Partial thromboplastin time (PTT)

Partial thromboplastin time (PTT) is a blood test that looks at how long it takes for blood to clot. It can help tell if you have bleeding or clotting problems.

Alternative Names

APTT; PTT; Activated partial thromboplastin time

How the test is performed

The health care provider uses a needle to take blood from one of your veins. The blood collects into an air-tight container. You may be given a bandage to stop any bleeding. If you are taking a medicine called heparin, you will be watched for signs of bleeding.

The laboratory specialist will add chemicals to the blood sample and see how many seconds it takes for the blood to clot.

How to prepare for the test

The health care provider may tell you to stop taking certain drugs before the test. Drugs that can affect the results of a PTT test include antihistamines, vitamin C (ascorbic acid), aspirin, and chlorpromazine (Thorazine).

Do not stop taking any medicine without first talking to your doctor.

How the test will feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Your doctor may order this test if you have problems with bleeding or blood clotting. When you bleed, the body launches a series of activities that help the blood clot. This is called the coagulation cascade. There are three pathways to this event. The PTT test looks at special proteins, called factors, found in two of these pathways.

The test may also be used to monitor patients who are taking heparin, a blood thinner.

A PTT test is usually done with other tests, such as the prothrombin test.

Normal Values

The normal value will vary between laboratories. In general, clotting should occur between 25 to 35 seconds. If the person is taking blood thinners, clotting takes up to two and a half times longer.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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